1. Field of the Invention
The present invention relates to a light emitting unit for emitting light and an image taking apparatus provided with the light emitting unit for performing image taking operation of a subject.
2. Description of the Related Art
As a light emitting unit for emitting light, a flashlight unit is known which has a light emitting tube for emitting flashlight and a reflector (reflecting cap) for reflecting flashlight emitted from the light emitting tube, and in which the angle through which flashlight is distributed is reduced by bringing the light emitting tube closer to the reflector or increased by moving the light emitting tube away from the reflector to perform adjustment for suitably irradiating a subject with the distributed flashlight according to a picture taking condition. Such a flashlight unit, however, requires a mechanism for moving the light emitting tube, which is a hindrance to reducing the size of the flashlight unit.
An arrangement is then taken into consideration in which a variable-focus lens such as one described below is provided in front of the light emitting tube instead of the mechanism for moving the light emitting tube, and in which the refractive index of the variable-focus lens is changed to perform adjustment for suitably irradiating a subject with the distributed flashlight from the light emitting tube. This arrangement eliminates the need for the mechanism for moving the light emitting tube and ensures that the light emitting unit can be implemented in a compact construction.
As a light emitting unit for emitting light, an auxiliary light emitting device for automatic focusing (AF) is also known which assists an AF function by emitting distance-measuring auxiliary light from a light source (light emitting diode (LED)) at the time of picture taking at a low luminance. Distance-measuring auxiliary light emitted from the AF auxiliary light emitting device is radiated to a subject and a focusing operation is performed on the basis of distance-measuring auxiliary light reflected by the subject. In this focusing operation, continuous AF processing based on a so-called “mountain climbing method” is performed. That is, an in-focus position is determined in such a manner that a focusing lens is gradually moved to a position corresponding to the maximum of an evaluated focus value while moving the focusing lens in small steps forward and rearward along the optical axis and checking the direction of increase/reduction in the evaluated focus value.
Ordinarily, an AF auxiliary light emitting device is placed with an offset from the optical axis according to a layout. Therefore, the direction in which distance-measuring auxiliary light is emitted from the AF auxiliary light emitting device intersects the optical axis. For this reason, the light emitting device has a drawback in that the focal length of the lens that can be covered by the distance-measuring auxiliary light is limited to a restricted region and a remote position cannot be reached by the distance-measuring auxiliary light. To overcome this drawback, a larger light source for emitting distance-measuring auxiliary light may be used. However, an increase in size of the light source is undesirable from the viewpoint of reducing the size of the AF auxiliary light emitting device and a problem that the power consumption is increased arises.
A method for adjustment may then be taken into consideration in which a variable-focus lens described below is provided in front of a light source and the refractive index of the variable-focus lens is changed to perform adjustment for suitably irradiating a subject with flashlight emitted from the light source. This method eliminates the need for a larger light source and enables implementation of an AF auxiliary light emitting device in a compact construction while limiting the power consumption.
As a variable-focus lens capable of changing the focal length, a liquid crystal lens capable of changing the focal length by using the electro-optic effect of a liquid crystal is known. For example, Japanese Patent Laid-Open No. 2002-341311 (hereafter referred to as a patent document 1) discloses a liquid crystal lens having first and second light-transmissive substrates in the form of a flat plate, a third light-transmissive substrate having two concave surfaces and provided between the first and second light-transmissive substrates, and a liquid crystal enclosed in each of a space between the first and third light-transmissive substrates and a space between the second and third light-transmissive substrates. In this liquid crystal lens, the orientation of liquid crystal molecules is changed according to the level of an applied voltage to change the refractive index of the liquid crystal lens. The focal length of the lens is thereby changed.
A fluid lens which can be changed in shape by application of a voltage to change its focal length is also known as a variable-focus lens. For example, a fluid lens in which an immiscible fluid constituted of a non-electroconductive oil and an electroconductive aqueous solution is enclosed in a tube having its inner wall surface covered with a water-repellent coating is proposed in “Philips' Fluid Lenses”, [online], Mar. 03, 2004, Royal Philips Electronics, [searched on Mar. 31, 2004], Internet <URL: ns.asp>(hereafter referred to as a non-patent document 1). In this fluid lens, when no voltage is applied, the aqueous solution constituting the immiscible fluid is a semispherical mass and the interface of the aqueous solution on the oil is convex. This interface changes between the convex state and a concave state according to the level of the applied voltage. Consequently, the radius of curvature of the lens can be changed and the focal length of the lens is freely variable.
In the technique disclosed in the patent document 1, the focal length of the lens is changed by using the difference Δn(n∥−nζ) between the refractive index (n∥) in the major-axis direction and the refractive index (nζ) in the minor-axis direction of liquid crystal molecules. However, there is a problem that the difference Δn is so small that the refractive index of the lens cannot be freely changed. In a case where this liquid crystal lens is used in the above-described flashlight unit, therefore, a problem arises that the angle through which flashlight is output cannot be freely changed. Also, in a case where the liquid crystal lens is used in the above-described AF auxiliary light emitting device, a problem arises that the direction in which distance-measuring auxiliary light is output cannot be freely changed.
In the technique proposed in the non-patent document 1, the focal length of the fluid lens is changed by applying a voltage to the immiscible fluid. When a voltage is applied to the immiscible fluid, a current flows through the electroconductive aqueous solution constituting the immiscible fluid. Therefore, there is a risk of the aqueous solution being decomposed by electrolysis to generate hydrogen and oxygen, and there is a problem that during use over a long time period a gas constituted of generated hydrogen and oxygen is accumulated to form bubbles which scatter light and deteriorate the performance of the lens. In a case where the fluid lens is used in the above-described flashlight unit, a problem arises that it is difficult to output light through an increased output angle with accuracy during use over a long time period. Also, in a case where the fluid lens is used in the above-described AF auxiliary light emitting device, a problem arises that it is difficult to output light in a direction with accuracy during use over a long time period.